>...Notice that the ground floors are all a dark color, whereas most of

the upper stories are light colored. Does anyone know why this is?

To better showcase storefront merchandise (or cafe patrons).

Think of evening "window shopping", when the street is dark, and the stores
remain lit, drawing the eyes deep inside. The building disappears---all
eyes are on the contents within.

A similar effect is approximated by using dark colors around storefront
windows: Where bright/light colors would create a stronger visual
separation between the street and the inside of the store (especially under
bright, glare-inducing sunlight), a dark frame draws you in. The shade of
awnings and arcades has a similar (delightful) effect.

>Does anyone know why there is a broad staircase leading into the ground?

I think this photograph was probably taken before the construction of the
Paris Metro. It might lead to public bathrooms. If it's neither of these,
then I'm baffled.

I can't explain the lack of cars in the photo, but this in fact the
Metro. I walked up those stairs and leaned on that railing for quite some
time when I visited Paris in '95. What a magnificent facade!

Sadly, this really isn't a square at all, but rather a huge, major
intersection, approached by streets from seven directions, with two
(not-so-circular) traffic circles helping direct all the cars. The other
traffic "circle", also a Metro entrance, is behind the photographer, on the
other side of the boulevard that roars through the middle of this "place".